A magnetisation and Mössbauer study of triazole (M_1−x²⁺M_x³⁺)M³⁺F₅(Htaz)_1−x(taz)_x weberites (M = Fe, Co, Mn, Zn, Ga, V)

Abstract

is obtained by hydrothermal synthesis. All phases are found to be isostructural with ZnAlF5(Htaz) by powder X-ray diffraction. Weberite structures are prone to induce the magnetic frustration of antiferromagnetic interactions originating from the cationic topology of HTB layers. The (nD) magnetic properties of (0D) Co-Ga, (1D) Zn-Fe, (3D) Fe-Ga, Mn-Fe, Co-Fe and Co-V couples, are thus reported. Co 2+ or Fe 2+ magnetic anisotropy induces a negative magnetisation below N T and compensation temperatures for Mn-Fe and Co-Fe couples. All iron 3D magnetic phases exhibit high Néel temperatures, between 81 K and 102 K, and large N P T /  ratii, signalling strong magnetic frustration. Their cation site occupancies and the deduced (de)protonation states of the amine are accurately determined by 57 Fe Mössbauer spectrometry. In addition, this spectroscopy evidences a subtle effect of the atmosphere that surrounds the samples: the magnetic ordering temperatures N T decrease significantly when the samples are cooled under vacuum with respect to samples that are cooled at ambient pressure. This novel phenomenon, that is highlighted for all studied (3D) triazole iron weberites, is reversible, and thus opens promising perspectives for understanding the underlying mechanism.

Strong magnetic exchange and frustrated ferrimagnetic order in a weberite-type inorganic–organic hybrid fluoride

“…Several prime examples are to be found within an extensive series of weberite-type inorganic-organic hybrid fluorides, M 2+ M 3+ F5(Htaz), where M 2+ = Mn, Fe, Co, Ni, Cu, Zn, M 3+ = Ti, V, Mn, Fe, Ga and Htaz is the organic linker 1,2,4-triazole. 13,14 These hybrid frameworks, isostructural with ZnAlF5(Htaz), 15 were recently reported by us to adopt an orthorhombic Imma structure at room temperature, closely related to that of the fluoride mineral weberite, Na2M 2+ M 3+ F7, [16][17][18] and Fe2F5(H2O)2. 19,20 The weberite structure is composed of intersecting and almost perpendicular hexagonal tungsten bronze (HTB) layers (Figure 1a), characterised by triangles of corner-sharing M 2+ and M 3+ octahedra (Figure 1b).…”

“…temperatures significantly higher than is typical of magnetic coordination frameworks [13]. Those based on cation groups of Fe 2+ /Fe 3+ /Ga 3+ , Co 2+ /Fe 3+ , Mn 2+ /Fe 3+ and Mn 2+ /Fe 2+ /Fe 3+ order magnetically at 80 K, 82 K, 100 K and 102 K, respectively, each with a net ferrimagnetic behaviour below T N .…”

“…Figure 4 compares the 57 Fe Mössbauer spectra collected at 12 K in zero field and under an applied field of 8 T. As with previous measurements at 77 and 300 K on this and other related samples within the M 2+ M 3+ F5(Htaz) series, the hyperfine data characteristic of Fe2F5(Htaz) at 12 K are consistent with a Fe 2+ /Fe 3+ ratio less than one. 13 This implies a small excess of Fe 3+ ions within the structure, substituted on the Fe 2+ 4c sites, which may be charge balanced by the deprotonation of the same fraction of Htaz linker molecules. In the present case, the Mössbauer data indicate that the substitution of Fe 3+ for Fe 2+ is on the order of 4 %, such that the precise chemical composition is (Fe 0.96 2+ Fe 0.04 3+ )Fe 3+ F5(Htaz)0.96(taz)0.04.…”

“…21 In our previous publication, we demonstrated -by magnetometry measurements and 57 Fe Mössbauer spectrometry -that four compounds within this series of weberite-type inorganic-organic hybrid fluorides with three-dimensional magnetic connectivity order magnetically at temperatures significantly higher than is typical of magnetic coordination frameworks. 13 Those based on cation groups of Fe 2+ /Fe 3+ /Ga 3+ , Co 2+ /Fe 3+ , Mn 2+ /Fe 3+ and Mn 2+ /Fe 2+ /Fe 3+ order magnetically at 80 K, 82 K, 100 K and 102 K, respectively, each with a net ferrimagnetic behaviour below TN. In the hybrid frameworks based on cations with strong magnetocrystalline anisotropy -Co 2+ and Fe 2+ -the direction of the magnetisation was observed to switch depending on the thermal and magnetic history of the sample.…”

“…For instance, there are a few cases of inorganic-organic hybrid framework materials that have sufficiently dense structures that the magnetic ordering of their paramagnetic metal ion centres occurs above liquid nitrogen temperatures. Several prime examples are to be found within an extensive series of weberite-type inorganic-organic hybrid fluorides, M 2+ M 3+ F 5 (Htaz), where M 2+ = Mn, Fe, Co, Ni, Cu, Zn, M 3+ = Ti, V, Mn, Fe, Ga and Htaz is the organic linker 1,2,4-triazole [13,14]. These hybrid frameworks, isostructural with ZnAlF 5 (Htaz) [15], were recently reported by us to adopt an orthorhombic Imma structure at room temperature, closely related to that of the fluoride mineral weberite, Na 2 M 2+ M 3+ F 7 [16][17][18], and Fe 2 F 5 (H 2 O) 2 [19,20].…”